FIG. 1 is a circuit diagram illustrating the configuration of a resonant-type multiple output switching power source apparatus according to a related art. In this multiple output switching power source apparatus, the primary side of a transformer T1 includes a full-wave rectifying circuit 2 to rectify an AC voltage from a commercial power source 1, a smoothing capacitor C3 connected between output terminals of the full-wave rectifying circuit 2, to smooth an output from the full-wave rectifying circuit 2, a first switching element Q1 and a second switching element Q2 (for example, MOSFETs) that are connected in series between ends of the smoothing capacitor C3, to receive a terminal voltage of the smoothing capacitor C3 as a DC input voltage sin, a control circuit 10 to control ON/OFF of the first switching element Q1 and second switching element Q2, a voltage resonant capacitor Crv connected in parallel with the second switching element Q2, and a series resonant circuit connected to both ends of the voltage resonant capacitor Crv.
The series resonant circuit consists of a primary winding P1 (the number of turns of N1) of the transformer T1, a reactor Lr, and a current resonant capacitor Cri that are connected in series. The reactor Lr is, for example, a leakage inductance between the primary and secondary sides of the transformer T1.
On the secondary side of the transformer T1, a first secondary winding S1 (the number of turns of N2) is wound to generate a voltage whose phase is opposite to the phase of a voltage of the primary winding P1 of the transformer T1, a first rectifying-smoothing circuit is connected to the first secondary winding S1, a second secondary winding S2 (the number of turns of N3) is wound to generate a voltage whose phase is opposite to the phase of the voltage of the primary winding P1 of the transformer T1, and a second rectifying-smoothing circuit is connected to the second secondary winding S2.
The first rectifying-smoothing circuit has a diode D1 and a smoothing capacitor C1, rectifies and smoothes a voltage induced by the first secondary winding S1 of the transformer T1, and outputs a first output voltage Vo1 from a first output terminal. The second rectifying-smoothing circuit has a diode D2 and a smoothing capacitor C2, rectifies and smoothes a voltage induced by the second secondary winding S2 of the transformer T1, and outputs a second output voltage Vo2 from a second output terminal.
This multiple output switching power source apparatus has a feedback circuit 5 to feed back to the primary side a signal corresponding to a voltage generated on the secondary side of the transformer T1. Namely, an input side of the feedback circuit 5 is connected to the first output terminal (Vo1), compares a terminal voltage of the smoothing capacitor C1 with a predetermined reference voltage, and feeds an error voltage as a voltage error signal back to the control circuit 10 on the primary side.
According to the voltage error signal fed back from the feedback circuit 5, the control circuit 10 alternately turns on/off the first switching element Q1 and second switching element Q2 thereby carrying out PWM control of controlling the first output voltage Vo1 to be constant. In this case, gates of the first switching element Q1 and second switching element Q2 receive control signals, i.e., voltages that may set a dead time of about several hundreds of nanoseconds. With this, the first switching element Q1 and second switching element Q2 do not overlap their ON periods with each other and are alternately turned on/off.
Operation of the multiple output switching power source apparatus according to the related art having the above-mentioned configuration will be explained with reference to waveforms illustrated in FIG. 2.
In FIG. 2, VQ2ds is a drain-source voltage of the second switching element Q2, IQ1 a current passing through a drain of the first switching element Q1, IQ2 a current passing through a drain of the second switching element Q2, Icri a current passing through the current resonance capacitor Cri, Vcri a terminal voltage of the current resonant capacitor Cri, ID1 a current passing through the diode D1, VN2 a terminal voltage of the first secondary winding S1, and ID2 a current passing through the diode D2.
The first output voltage Vo1 is controlled by the control circuit 10 that receives the voltage error signal fed back to the primary side from the first rectifying-smoothing circuit through the feedback circuit 5 and carries out PWM control on the first switching element Q1. In this case, the first switching element Q1 and second switching element Q2 are alternately turned on/off with a dead time of about several hundreds of nanoseconds in response to control signals from the control circuit 10, as mentioned above.
First, in an ON period (for example, time t11 to t12) of the first switching element Q1, the current resonant capacitor Cri accumulates energy through an exciting inductance of the primary winding P1 of the transformer T1 and the reactor Lr (leakage inductance between the primary and secondary sides of the transformer T1).
Next, in an ON period (for example, time t12 to t14) of the second switching element Q2, the energy accumulated in the current resonant capacitor Cri causes the reactor Lr and current resonant capacitor Cri to pass a resonant current and send energy to the secondary side. Also, the exciting energy of the exciting inductance of the primary winding P1 is reset.
More precisely, in the ON period of the second switching element Q2, the primary winding P1 receives a voltage that is produced by dividing the terminal voltage Vcri of the current resonant capacitor Cri with the exciting inductance of the primary winding P1 and the reactor Lr. When the voltage applied to the primary winding P1 reaches (Vo1+Vf)×N1/N2, it is clamped and the current resonant capacitor Cri and reactor Lr pass a resonant current to send energy to the secondary side. This results in passing the current ID to the diode D1. If the voltage of the primary winding P1 is smaller than (Vo1+Vf)×N1/N2, no energy is transmitted to the secondary side of the transformer T1 and the exciting inductance of the primary winding P1 of the transformer T1, the reactor Lr, and the current resonant capacitor Cri conduct a resonant operation only on the primary side. Here, Vf is a forward voltage drop of the diode.
In general, the ON period of the second switching element Q2 is determined by the ON period of the first switching element Q1 under a fixed frequency, or it is an optional fixed period. Changing the ON period of the first switching element Q1 to change the duty ratios of the first switching element Q1 and second switching element Q2 results in changing the voltage of the current resonant capacitor Cri, and therefore, it is possible to control the quantity of energy to be sent to the secondary side.
The first secondary winding S1 and second secondary winding S2 are coupled with the same polarities. Due to this, while energy provided by the first secondary winding S1 is being output as the first output voltage Vo1 in an ON period of the second switching element Q2, energy provided by the second secondary winding S2 is output as the second output voltage Vo2. This second output voltage Vo2 is nearly equal to Vo1×N3/N2.